The present invention is an improvement in an electrical device which, since its introduction many years ago, has become an industry standard while remaining essentially unchanged from its original design. In particular, the invention relates to an electrical connecting device commonly known as a "phone plug" which terminates an electrical cable and permits the cable to be readily connected and disconnected to electrical apparatus. One of the characteristics of the phone plug that has elevated it to a standard piece of equipment for terminating an electrical cable, is the elegant simplicity of its design. The phone plug, which has no moving parts, is sturdy and sure in performing its function permitting it to be connected and disconnected countless times without failure.
In addition to its many other applications, the phone plug has become the standard device by which electrically amplified musical instruments are connected to their amplifiers. And while the phone plug fully performs its function of easily connecting and disconnecting a cable to a musical instrument, it does so with a most undesirable side effect--a high pitched squeal or screech if the amplifier is on at the time, which it frequently is. Although a number of devices have been proposed for eliminating these nerve wracking screeches, no device prior to the present invention has been effective in doing so without at the same time compromising the integrity of the plug as an electrical connecting device. For that reason, phone plugs in use today remain essentially unchanged from their initial design.
The phone plug is characterized by an outer, tubular, cylindrical shaft terminated at one end by a tip member which is connected to a cylindrical shaft which is co-axially aligned within the outer shaft, but electrically insulated therefrom, as is the tip. A cable terminated by a phone plug has its signal carrying conductor electrically connected to the inner shaft member (and thereby the tip), while the outer shaft is connected to the cable ground conductor. When the phone plug is inserted into a mating jack, the tip engages and is held in place by a contactor electrically connected to the instrument's signal generating circuit, while the outer shaft is grounded. These mechanical and electrical features of phone plugs and jacks are well known in the art and have, as mentioned above, become standard, universally used components.
What is also universal, however, is the highly undesirable noise that occurs when a phone plug is connected or disconnected from an instrument while the instrument's amplifier is on. Under those circumstances, it is virtually impossible to either insert or withdraw the phone plug from its jack without the tip, which carries a voltage, from contacting some signal generating element, whether it be part of the jack, the person handling the equipment, or some other nearby object. When the tip makes such contact, the result is amost annoying screeching sound emanating from the speakers being driven by the amplifier to which the cable is connected.
Ever since phone plugs and musical instruments have been used together, the screeching that occurs when the instruments are connected to or disconnected from a "live" amplifier has been a common occurrence, and yet, prior to the present invention, all attempts to solve this problem have been unsatisfactory as either too complicated, too costly or too compromising of the plug as a connector, or some combination thereof. Two such solutions are described in U.S. Pat. Nos. 4,275,946 and 4,275,947. Both of these patents recognize that the solution to the problem is to ground the tip, except when it is engaged in the jack and thereby prevent any cacophonous amplified signal from being delivered to and broadcast from the speakers. The devices taught by these two patents are, however, so complex compared to the simplicity of the phone plug that they compromise the integrity of the phone plug as a reliable means for easily connecting and disconnecting cables to equipment. In both of the aforementioned patents, for example, the outer, tubular shaft of the phone plug which is grounded, and the inner coaxially disposed shaft, which carries a voltage are slidably disposed with respect to one another where, in the standard phone plug, these two elements are not movable at all. These moving members in the aforementioned prior art patents, as well as the springs and other components not found in a standard phone plug render the plug expensive to manufacture and unreliable in operation. For these reasons alone, these prior art devices, as well as others of the same general description, have never enjoyed any commercial success.
The present invention, by contrast, provides a solution to the problem which is as elegant as the simplicity of the phone plug itself and which in no way compromises the effectiveness of the phone plug as an electrical connecting device.
Accordingly, it is an object of the present invention to provide a means for eliminating the screeching sound that occurs when a phone plug is connected to or disconnected from a musical instrument.
It is another object of the present invention to provide a means for grounding the end of a phone plug at all times other than when it is seated in a phone plug jack.
It is yet another object of the present invention to provide a switch which can be disposed between the outer and inner shafts of a phone plug so as to ground the tip of the plug except when it is seated in a plug jack.
It is another object of the present invention to provide a simple and reliable method of securing a switch between the outer and inner shafts of a phone plug in perfect alignment with a hole in the plug's outer shaft.
Other objects and advantages of the present invention will be made apparent from the summary of the invention and the detailed description of the invention which follows.